If you've noticed that your internet connection is slow since you installed Tiger, you're not alone. On both of my machines, OS X 10.4 effectively killed my internet connection. I was averaging 100kpbs download, which is very slow compared to normal. I also couldn't do video chats with anyone on iChat unless I tried over and over, and the quality was always awful. Today I found the issue:

Open System Preferences - Network. Go into the properties for your connection and click on the TCP/IP tab. Then click the Configure IPv6 button and set the menu to Off. Click OK and then Apply Now.

In about 10 seconds, your network will hopefully be speedy again. I went from 100kpbs to 700kpbs and I was able to do a 4-way video chat. IPv6 was definitely my issue, and it might just be yours if you've been having internet speed issues.

[robg adds: My IPv6 is clearly enabled, but I haven't noticed any speed loss from 10.3 to 10.4. Still, if you have, this might be worth trying...]

The reason why some may notice a speed improvment by disabling ipv6 and other users aren't it's cause the speed boost is due to hardware that doesn't do well with ipv6 (probably your network router if you're in a small home network).

I have a draytek vigor 2600 router and I have that problem especialy in name resolution (DNS), if you don't notice a speed improvment by disabling the ipv6 then you can activate it without any problems cause all your hardware suports it fine!

Turned IPv6 off on my G5 iMac and my MacBook...
New speeds on OSX: 2mbps to 4mbps average

Then I tweaked use of "automatic" DNS with Comcast, OpenDNS and Verizon's famous 4.2.2.1 DNS set.

Comcast auto DNS with OSX: 1.5mbps to 3.4mbps
OpenDNS with OSX: 2mbps average
Verizon DNS with OSX: 3mbps to 4mbps average
Comcast auto DNS with XP: 3mbps to 4mbps
OpenDNS with XP: 5mbps average
Verizon DNS with XP: 6mbps to 10mbps average

I still don't know why my iMac and MacBook are getting such crappy speeds when hooked up to the very same router and Comcast modem as my XP system. Why would OSX have such terribly slow test speeds compared to the SAME setups as XP?

IPv6 helped, but it didn't solve.

P.S. tried all the above with wireless and got the same XP speeds, but even slower OSX speeds. Tried with XP running via BootCamp on my MacBook and still got awesome wireless download speeds.

Turning IPv6 off gave me a tiny but noticeable performance increase. That's nice, but nowhere near enough. I'm paying for 8mbps service, and I'm not even getting 1/4 of its potential.

After talking to someone very smart at Apple and someone (average) at Motorola, I found out that the Motorola SB5101 cable modem is not fully compatible with "certain" Mac systems. See my voicemail link below.

I tried a new Linksys cable modem, but it had the same problems - crappy download speeds, but decent upload speeds. I finally switched to the SB5120 and now I'm getting 10mbps speeds!!

From a PC laptop hooked up to a lowly 10 MB PCMICA Ethernet card, I consistently get:
3.54 Mbps down and 361 Kbps up

On my mini from either the AirPort or 1000BaseT, I got:
1.99 Mbps down and 361 Kbps up

I executed sudo sysctl -w net.inet.tcp.recvspace=65536 and got:
2.10 Mbps down and 361 Kbps up

I also disabled IPv6 and got:
2.22 Mbps down and 361 Kbps up

Still there's a big discrepancy (1/3 or better!) between download speed on a PC and my Mini (Aluminum 15" PowerBook gave similar results) and I'm trying to determine what other parameters might help. Any thoughts are appreciated.

I know this thread is four years old but I thought I'd share something I found: the reason Windows gets better network speed than OSX is because flow-control is enabled by default in OSX. Try disabling it and you should see a speed increase: System Preferences -> Network -> Advanced -> Ethernet -> Configure Manually -> Change "Duplex" setting from "full-duplex, flow-control" to just "full-duplex".